1. Field of the Invention
This invention relates to an incremental motion control system for controlling the position of a motor. More particularly, this invention relates to a paper feed system for use with high speed line printers, i.e., those which print at a rate of greater than 1,000 lines per minute.
High speed line printers generally employ a type font carried on a band which moves horizontally across the paper being printed. A hammer bank impacts against the moving band to cause printing. In order to increase the throughput of the printer, either the paper feed time (the time to move the paper from one print line to the next) and/or the print time (the time it takes to actually print a line) must be decreased. A reduction in print time is generally undesirable, since it also brings about a reduction in print quality. This is due to the fact that in order to reduce print time, the font speed must be increased, resulting in smearing of the characters. Therefore, it is desirable to provide as much time as is possible to accomplish actual printing. This means that paper feed time should be reduced to an absolute minimum.
2. Description of the Prior Art
Several methods have been utilized in the past to rapidly increment paper for a high speed line printer. One method involves the use of an open loop step motor, driven by any number of drive schemes, and incremented by a fixed timing sequence. This type of motor requires a torque margin to prevent a loss of steps, and therefore the full torque of the motor cannot be utilized. In addition, the motor is sensitive to load variations and is susceptible to resonances. Such a motor is very under-damped and will ring when positioning.
A second type of method uses a closed loop servo control system along with a DC brush type motor. A DC generator or an electronic tachometer is used for velocity feedback, and a position encoder is sometimes used for position feedback loops. The motors used in this type of system are relatively inefficient, and generally require undesirable forced air cooling for medium to high speed systems. Because of the motor's inefficiency, the system consumes a great deal of power.
Yet another type of system utilizes a clutch and brake to incrementally couple the paper feed mechanism to a continuously rotating shaft. Such systems are generally quite complex and are prone to misadjustment and unreliable performance. In addition, these systems are relatively expensive when used for high speed printers.
Some attempts have been made to utilize a step motor in a closed loop system for incremental motion. These previous attempts have not been able to deliver the high performance necessary to increment very rapidly. In addition, the damping of such systems has generally been quite poor.
One prior art system is disclosed in U.S. Pat. No. 4,228,396 to Palombo, et al. This system utilizes a brushless DC motor driven in a servo controlled fashion. Other paper feed control systems are disclosed in U.S. Pat. Nos. 3,524,528 to Peyton and 3,656,041 to Bonzano.